Cons

Expensive.

Long recharging time.

Attacks are moderate against enemies with traits.

Freezing time is extremely short (but freeze also isn't the main purpose of this cat)

Slow movement speed.

Strategy/Usage

Shadow Gao is a mighty being capable of rending a mountain in twain; Diabolic Gao is He Who Surpasses The Heavens. With an outstanding 15.6k DPS against the most common enemy type in the game prior to Hypermaxing, as well as Shockwave immunity and 100% Freeze chance (if briefly), Diabolic Gao unleashes unspeakable power and fury onto all who dare stand before you. Not Le'Noir nor Manic Eraser Cat can withstand his (meatshielded) might, as his bitter frost scatters all to the wind and into oblivion.

Diabolic Gao rules over a very narrow niche in the Cat Army: Massive damage against non-traited enemies. As they are by far the most common enemies in the game, Shadow Gao is an incredible offensive force able to wipe out many enemy lineups. Diabloic Gao is even better, as it costs the same, gets a mild stat boost, and gains the ability to momentarily freeze traitless foes. This doesn't last for long, but given Diabolic Gao's good attack speed it is more useful, and either way Diabolic Gao's style involves utter annihilation, not slow attrition so this is just a small bonus.

Shadow Gao will make dealing with enemies such as Master A. and R. Ost utterly trivial, as long as he is protected by meatshields. Note that Shadow Gao is also immune to Shockwaves, making him Kory's worst nightmare. He will also make Floor 30 of the Heavenly Tower pretty easy.

His use is not limited to annihilating traitless enemies; he can also be used as a powerful offensive support unit to clear out peons surrounding a traited enemy, giving more specialized units an unobstructed attack. When used in tandem with Amaterasu or Mighty Lord Gao, there is almost nothing that can stand in their way.

Even at his worst, Diabolic Gao can be used as a superior Bahamut; though the price tag jump is high, the impressive un-hypermaxed 5.2k DPS combined with fast attack rate (Bahamut has under 4k DPS) and also shockwave immunity makes Diabolic Gao almost a direct upgrade to Bahamut.

(Note that Mighty Lord Gao has better DPS against traited foes, and also takes half damage. He also benefits from the Anti-Trait treasures, while Shadow Gao doesn't)

Appearance

Normal Form: Similar in appearance to his normal counterpart, Mighty Lord Gao, but he is now jet black with the exception of the blue spirals on his crest and shoulders. He is somewhat smaller than normal Gao, and the crest over his head has decreased in size as well, but there's still a Cat riding atop him.

Evolved Form: Gao now has more blue detailing across his body, including his crest which became more ornate. He seems to have gained somewhat of a charcoal colored mane and a second tail. The normal cat is still present on top of Gao's neck/back.

Trivia

When he was released, PONOS changed enemies that are both Floating and White/Grey into only Floating, such as Teacher Bun Bun, to prevent Shadow Gao from being too overpowered.